Simplified the way a Promise is tested for (removed promiseMarker). Removed isCancelled helper method from _Deferred, wasn't used nor tested. Reworked jQuery.Deferred and removed unnecessary variables. Also ensured a Promise will return itself when asked for a Promise. Finally, the jQuery.when tests have been revamped.

Introduced a new promise method on deferreds that returns an immutable object (exposing then, fail, isResolved, isRejected and promise itself only). Remove $.isDeferred and moved logic directly into $.when. Made sure $.when returns a promise by using promise(). Used promise() in ajax code too.

Removed re-usability from jXHR object (no more open, send & onreadystatechange support). Streamlined the implementation and put it back into ajax.js (removed xhr.js in the process). Went back to a more simple & direct approach to options handling (keeping much room to further minification-related optimizations). Code is cleaner, smaller & faster. Removed & edited unit tests accordingly. All build files have had xhr.js removed.

Simplified the way a Promise is tested for (removed promiseMarker). Removed isCancelled helper method from _Deferred, wasn't used nor tested. Reworked jQuery.Deferred and removed unnecessary variables. Also ensured a Promise will return itself when asked for a Promise. Finally, the jQuery.when tests have been revamped.

Introduced a new promise method on deferreds that returns an immutable object (exposing then, fail, isResolved, isRejected and promise itself only). Remove $.isDeferred and moved logic directly into $.when. Made sure $.when returns a promise by using promise(). Used promise() in ajax code too.